Interesting idea, and well done so far
I have very little idea whether this will be fun or frustrating.
Comments on gameplay:
These stairs are horrible at clipping the player. From some angles they stop you entirely.
Are you supposed to be able to jump over this wall? It's currently quite easy to jump over, except that it's harder in some places than others. Also, because of the sloping section, it is possible to jump/trimp up the large wall at the front of the base.
I hate to suggest this, but you might want to put a sniper bunker in, on top of the hill and maybe off to one side. This could give a sniper some protection. The map is so small they might not have much of a chance otherwise. At the very least, this seems like it could easily be a nice small sniper map.
Mapping Issues:
Alpha painting! You don't have to have the same floor texture over all of the island, you can use alpha painting to seamlessly mix different textures on a displacement. So you could have say a few muddy or grassy patches, whatever you want.
Lighting inside looks like it is fullbright, which is definitely a bad thing. Just play around until you get something plausible and nice.
Couple of bugs which you may have spotted already:Floating TreeUnsewn Displacements

The island, while a nice idea, seems like more trouble than it's worth. As you said, getting the water to transition from expensive to cheap at the right points to match with the 3D and 2D skyboxes can be a real bitch. Why not just scrap the idea of an island in the middle of an ocean, and instead make it an island floating in the middle of the sky? Not having to deal with rendering the water could help people's performance, especially on slower computers. Additionally, blowing people off the edge of the map could be pretty fun. I just imagine a floating island looking something like this, as seen from the side:

Crap, in the time it took me to make that with symbols, I probably could have drawn and colored you a picture... anyway I know you already made a map called Sky, so maybe you'll like the idea. What do you think?

That does sound pretty groovy - I like the idea of blasting people off the side of the map to their doom. Right now I like the islandy aesthetics of Attrition, but I feel like the gameplay would be more rediculous and Fortress-y if it was an island floating in the sky. I'll revisit it in a while, but right now I'm relatively unmotivated

As your facing the blue base thwe water to your right is what I'm refrencing to.

as your coming from the water to the center area, I noticed that you can see the skybox in the ground at the top of the heal, also I don't know if you can blend textures or not but in that general area you can see lines in the ground.

intead of having the ocean visible from all sides maybe you could add some big hills around three quarters of the island, just enough to cover the horizon. this way players could go around them if they wanted, and the fps shouldnt take too much of a hit for em because its just hills on one side, not the whole map.

just a thought because i like the real island idea better than a floating one

Yep, just big enough to hide the horizon when your player is on most of the map. You will have less lag because the computer doesnt have to render the horizon clear across your screen at all times when you are outside.